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A series of marginal-sea basins and fault-depression basins were formed in eastern China under the background of subduction of the West Pacific plate. Different types of helium-rich natural gas reservoirs (He>1000 ppm, 1 ppm=1 µmol mol−1) have been found in these basins: helium-rich CO2 gas reservoirs, helium-rich N2 gas reservoirs, and helium-rich hydrocarbon gas reservoirs. Based on the analysis of gas geochemical data, the source and accumulation mechanism of helium in these helium-rich natural gas reservoirs were discussed. Helium-rich natural gas has relatively high 3He/4He ratios (0.88–4.91 Ra, average 2.82 Ra). The 3He/4He ratio characteristics of mantle xenoliths and mantle-derived CO2 gas reservoirs indicate that the helium in these helium-rich natural gas reservoirs is mainly mantle-derived (>70%). The original mantle volatile is mainly CO2 with a low helium concentration (He<200 ppm), and the enrichment of mantle-derived helium in the gas reservoir is mainly related to the dissolution and mineralization of CO2. During this process, the CO2/3He ratio decreases from 2×109 to approximately 2×106. As CO2 dissolves and mineralizes, the concentration of conservative gases (He and N2) increases in the remaining CO2 gas proportionally to the loss of CO2. Large amounts of carbonate minerals, such as dawsonite, which are relatively enriched in 13C, are found in CO2 reservoirs in eastern China. The relative enrichment of 12C in residual CO2 gas is important evidence of the dissolution and mineralization of CO2. The relative abundance of mantle-derived helium and N2 gas increases thousands of times during the dissolution and mineralization of CO2, which is the main accumulation mechanism of mantle-derived helium-rich CO2 gas reservoirs and helium-rich N2 gas reservoirs. Helium-rich gas from the mantle is mixed with alkane gas generated by organic matter in the sedimentary basin to form helium-rich hydrocarbon gas reservoirs. [ABSTRACT FROM AUTHOR]

• The carbon isotope curve index (CICI) summarizes the isotope ratios of C 1 - C 3. • Carbon-isotope curves can be divided into Convex, Linear, Concave and Reversed. • In Normal-type gas, leakage and preferential propane biodegradation increase CICI. • In Normal-type gas, migration and mixing with microbial methane decrease CICI. The carbon isotope curve (δ13C n versus 1/n) is often used to evaluate the origin and secondary alteration of hydrocarbon gases, but the factors that influence the carbon isotope curve are not fully understood, limiting its practical application. We define a carbon isotope curve index (CICI) to quantitatively characterize the curve type. Four general curve types are defined: a Normal (δ13C 1 < δ13C 2 < δ13C 3) type and three Reversed types consisting of R 1 (δ13C 1 > δ13C 2 > δ13C 3), R 2 (δ13C 1 < δ13C 2 > δ13C 3), and R 3 (δ13C 1 > δ13C 2 < δ13C 3). CICI is defined as arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for Normal and R 2 , –180 + arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for R 1 , and 180 + arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for R 3. CICI ranges from 0 to 90 for Normal, –180 to –90 for R 1 , –90 to 0 for R 2 , and 90 to 180 for R 3. Normal gases can be divided into three categories: Linear (CICI = 45), Convex (CICI 0 to < 45), and Concave (CICI > 45 to 90). The influence of kerogen type and maturity on CICI is investigated by comparing thermogenic gases generated from different kerogens in both sedimentary basins and pyrolysis experiments. Curves for low-maturity lacustrine and low-to-high-maturity coal gases are mainly Convex. In contrast, curves for high maturity to overmature marine gases are more Concave. The curve type of thermogenic gas changes from Convex to Concave with increasing maturity, and the mixing of gases generated from kerogen-cracking and oil-cracking results in the Concave type. The effect of post-generation alteration processes on CICI was elucidated using data from three field studies. In Normal-type gas, preferential biodegradation of propane and leakage increase CICI, while migration and mixing with microbial methane decrease CICI. Preferential biodegradation of propane and leakage are two other reasons for the Concave type. [ABSTRACT FROM AUTHOR]

Helium is an important, scarce, and strategic resource that is critical to national security and advanced technology development. Although helium originates from mantle degassing and the radioactive decay of uranium (U) and thorium (Th) in minerals, commercial helium resources are primarily derived from natural gas reservoirs with He concentrations higher than 1000 ppm. The gas geochemistry characteristics of 920 natural gas samples from nine petroliferous basins in China were used to classify and discuss the enrichment mechanisms of helium. Helium-bearing natural gas reservoirs can be divided into the following three types: basement source-migration-rich accumulation, self-generating and self-preserving, and crust–mantle complex types. The basement source-migration-rich accumulation, which is the most common type of proven helium-rich natural gas reservoir, occurs primarily in inherited anticlines and dome traps located on basement uplifts. Ancient shale systems demonstrate the potential to form helium-rich shale gas that belongs to the self-generating and self-preserving type. Crust–mantle complex helium-rich natural gas reservoirs develop in petroliferous basins in active magmatic–tectonic areas. The origins of helium and hydrocarbon gas are different, but the entrainment of helium during hydrocarbon gas accumulation is a key factor influencing the formation of helium-rich natural gas reservoirs. The nitrogen concentration in helium-rich gas samples is often extremely high, indicating that the nitrogen and the helium are derived from the same source rock or both are extracted from water by migrating gas. • Helium-rich gas includes basement source-migration-rich accumulation, self-generating and self-preserving, and crust–mantle complex types. • Exsoluion gathering of underground dissolved helium is the key mechanism of helium enrichment. • High-quality matching between helium and natural gas is crucial for the helium-rich gas accumulation. [ABSTRACT FROM AUTHOR]